A driver circuit for actuating print wire solenoids

ABSTRACT

A driver circuit for energizing solenoids to actuate print wires wherein the solenoid operation is such as to rapidly move the print wire into impact direction and permit rapid return thereof. The drive circuit provides for an external determination of the solenoid drive time. A transistor having a current limiting resistor coupled thereto drives the solenoid. Current amplification means is coupled to the imput of the transistor and its input, in turn is controlled by a voltage regulating semiconductor device for maintaining constant current through the solenoid. Capacitance means coupled to the inductor through diode means provides a discharge circuit for the solenoid. A capacitor cooperates with the solenoid inductance to form a resonant circuit enabling the current through the solenoid to decrease in a short time period and prevent generation of high voltages during reverse conduction of the inductance. Control circuit means preventing energization of the solenoids during cutoff of the solenoid power source.

Howard DRIVER CIRCUIT FOR ACTUATING PRINT WIRE SOLENOIDS Aug. 21, 1973Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr.Attorney0strenk, Faber, Gerb & Soffen [75] Inventor: Robert Howard,Roslyn, N.Y.

[73] Assignee: Centronlcs Data Computer 1 Corporation, Hudson, NH. [57]ABSTRACT Filedl, y 1972 A drivercircuit for energizing solenoids toactuate [21] AppL 252 616 print wires wherein the solenoid operation issuch as to rapidly move the print wire into impact direction and Rellted15- Appli a ion 8 permit rapid return thereof. The drive circuitprovides [60] Division of Ser. No. l52,598,Junel6,1971,Pat.No. for anexternal determination of the solenoid drive 3,670,431, which is acontinuation-in-part of Ser. No. time. A transistor having a currentlimiting resistor couy 1970- pled thereto drives the solenoid. Currentamplification means is coupled to the imput of the transistor and its[52] US. Cl. 317/1485 B, 3 l7/DlG. 6 i t, i turn is controlled by avoltage regulating semi- [51] Int. Cl. H0lh 47/32 conductor device formaintaining constant current F1616 131G- through the solenoid.Capacitance means coupled to 148-5 R the inductor through diode meansprovides a discharge circuit for the solenoid. A capacitor cooperateswith Reference! C1161 the solenoid inductance to form a resonant circuiten- UNITED STATES PATENTS abling the current through the solenoid todecrease in 3 049 650 8/l962 Greenblatt 3l7/DIG. 6 a time P?riod andPrevent generatim high 2:980:369 4/1961 Ruof 317/1485 R ages duringreverse conduction of the inductance 3,141,530 7/1964 Morley 317/1485 Rrol ircuit means preventing energization of the sole- 3,343,046 9/1967Ladd, .Ir 317/1485 R noids during cutoff of the solenoid power source.2,997,632 8/l96l Shepard, Jr 3l7/DlG. 4

3 Claims, 11 Drawing Figures 7 /2 6 20 6 WWW/M 7 H i L-Jl I ////l1/////JZT/Z/AA Z F ll 12 /2( l T \\\\\\\\\\\\Y\\\\ 1\ r Patented Aug. 21, 19733,754,166

5 Sheets-She t 1 v i f: -//c- Y //a //6 [/e I v 1 Patented Aug. 21, 1973 3 Sheets-Sheet 11 Patented Aug. 21, 1973 3,754,166

3 Sheets-Sheet 5 DRIVER CIRCUIT FOR ACTUATING PRINT WIRE SOLENOIDS Thisis a division of parent application Ser. No. 152, 598, filed June 16,1971 now U.S. Pat. No. 3, 670, 431, which is a continuation-in-part of,and an improvement upon copending application Ser. No. 37, 815 filed May15, I970. The present invention relates to solenoid drive circuits, andmore particularly to drive circuits useful for driving print wiresolenoids employed in dot matrix type printing devices wherein thesolenoids are driving by high constant current and abruptly turned offduring deenergization thereof without producing high voltages upondeenergization of the solenoids.

The solenoid assembly to be described hereinbelow in greater detail, isextremely advantageous for use in the high speed dot matrix printerdescribed in the inventor's copending application Ser. No. 35,405 filedMay 7, 1970. The printer described therein is capable of formingcharacters by selectively impacting one or more of seven verticallyaligned slender print wires against an inking ribbon so as to print dotsupon the surface of the paper document. A plurality of the columnsprinted are related so as to form any characteror symbol, typicallywithin a five by seven dot matrix. Obviously, another size matrix may beemployed, if desired. Since printing speed is of the essence in suchdevices it is therefore very important to provide an apparatus forrapidly impacting the print wires and rapidly returning the print wiresto the rest position with a minimum of bounce in order that the entireprinting assembly may operate at such high speeds. The printing rate inthe above mentioned printing apparatus is of the order of 165 charactersper second with 60 lines of characters per minute being printed (witheach line containing up to 132 characters). To obtain these speeds it isimportant to provide solenoid assemblies which are capable of moving theslender print wires from the rest position to the impact position andreturn to the rest position within the time interval of one millisecondor less while at the same time providing an impact of sufficient forceto print a clearly legible dot upon the paper document or other printingsurface.

The present invention is characterized by providing driving circuit forsolenoid assemblies. l

The solenoid assembly of the present invention comprises a case forhousing the coil and armature. A stem portion threadedly engages theforward end of the housing and is provided with an axially alignedelongatedopening for receiving and reciprocally mounting a slenderelongated print wires. The rearward end of the print wire is fixedlysecured to the solenoid annature at the forward end of the armature. Therearward end of the armature is fixedly secured to a spring member of'the wagon wheel type at its central portion. The peripheral portions ofthe spring rest against an an- 1 gular shaped spacer which abuts ashoulder provided therefor within the solenoid housing. The rearwardportion of the armature housing abuts a cap member which threadedlyengages and seals the rearward end of the housing. The cap member isadjustable to regulate the amount of stress or tension (i.e.,preloading) imposed upon the spring member. A half spoolmemberpositioned within the solenoid housing cooperates with the sternmember'to define an annular shaped hollow interior space for positioningand receiving the solenoid coil. The exposed forward end of the stemportion is threaded so as to threadedly engage an appropriate tappedaperture provided in the solenoid mounting portion of the print headassembly. A tubular wire guide is positioned within the solenoidassembly stem portion and is appropriately lubricated so as to reducewearing of the reciprocating print wire extending therethrough.

The print head assembly is provided with a plurality of guide means forguiding and directing each of the print wires toward the forward end ofthe print head housing which is further provided at its forward end witha multi-apertured jeweled member which ultimately aligns the forwardmostends of the slender print wires along a vertical straight line. I

The wagon wheel spring means is formed from a flat sheet of prehardenedtempered spring steel or any other suitable material exhibiting similarcharacteristics. The spring has a centrally located hub portion havingradially aligned spoke portions extending outwardly therefrom andintegrally joined therewith. Each of these spoke portions are providedwith arcuate shaped portions integrally joined to their free ends so asto define a substantially circular shaped periphery wherein gaps areprovided at discrete intervals about the periphery of the spring, whichgaps define the spacing between the outer ends of adjacent arcuateportions. The arcuate shaped portions rest against the spacer memberreferred to hereinabove while the hub portion of the spring rest againstthe forward end of the threaded cap member which is adjustablypositioned relative to the rearward end of the solenoid housing so as toprestress or preload the spring member and thereby provide the preciseamount of loading required to assure appropriate high speedreciprocating operation of the print wires under the control of thesolenoid.

The springs maintain the wires of their associated solenoids in thenon-printing position when the solenoid coil is deenergized. Uponenergization of the solenoid coil, the electromagnetic field tends tourge or pull the armature towards the forward end of the solenoidassembly. This force is counteracted by the spring member whichsequentially develops two counteracting spring forces in the followingmanner:

The initial forward movement of the solenoid armature causes each of theradially aligned spokes to flex, which flexing establishes acounteracting spring force which is substantially linearly related tothe movement of the annature. Subseuqently thereto, as the armaturemoves still further in the forward direction, the arcuate portions ofthe spring-provided at the free ends of the spoke portions experience atwisting. The resultant counteracting force developed by the spring thusfollows a logarithmic curve whereby the magnitude of the counteractingforce increases more rapidly than the otherwise linear force developedby the radial portions of the spring alone. The initial counteractingforce is both small and relatively linear in nature to enable thesolenoid annature to be rapidly accelerated. The counteracting force ofthe spring reaches its maximum magnitude as the slender print wiresimpact the paper document at which time the dots of the character areprinted. The deenergization of the solenoid coil places the print wires,which are now in the impact position, under the exclusive control of thecounteracting force of the spring which is substantially at its maximummag- Y nitude at this time causing the print wire to rapidly return tothe rest position. The flexed spokes, in returning to their normalundeflected position, tend to overshoot their normal rest position.However, the "twisted arcuate portions tend to counteract theovershooting and thereby more rapidly return the annature and print wireto the rest position with a minimum of overshoot and bounce.

The threadedly engaging members of the solenoid assembly permit simplerapid assembly of the component parts after which precise adjustmentsmay be made whereupon the adjustable members are then epoxied intoposition so as to maintain the accurate positioning of the componentsthereby providing a solenoid assembly which is formed of componentswhose tolerances may be somewhat imprecise while at the same time arecapable of being readily and easily adjusted to produce a solenoidassembly whose operating characteristics lie within highly precise andcontrolled ranges. The print head housing supports sensing means forprecisely registering the lines of dots printed.

The driver circuit for actuating the print wire solenoids utilizes atransistor coupled in circuit with the solenoid coil, a resistor and aDC source. A second transistor coupled as a current source drives thebase of the first transistor. The input of the second transistor isprovided with voltage regulating diode means coupled to its input. Thefirst and second transistors together with the voltage regulating diodemeans function to maintain a constant current through the solenoid coilduring energization thereof. Series connected second diode means and acapacitor are coupled between the solenoid coil and ground potential forrapidly dissipating the current in the solenoid in the solenoid coilduring deenergization thereof and serve to prevent the generation ofhigh voltages upon deenergization. The capacitance means has a capacityvalue which functions to form a tuned circuit with the inductance of thesolenoid coil to permitdecrease of current through the solenoid coil.

It is therefore one object of the present invention to provide a noveldriver circuit for solenoid assemblies employed in high speed dot matrixprinters and the like wherein the driver circuit is designed to providea large constant current to the solenoid coil during energizationthereof and is further provided with means for providing rapiddeenergization of the energizing currents.

Another object of the present invention is to provide a novel solenoidassembly for use in dot matrix printers and the like in which thecomponents are made adjustable so as to facilitate ease of assembly andsubsequent precise adjustments thereof.

Another object of the present invention is to provide a spring member ofwagon wheel" design for use in solenoid assemblies. 1

These as well as other objects of the present invention will becomeapparent when reading the accompanying description and drawings inwhich:

FIG. 1 is a sectional view of a solenoid assembly designed in accordancewith the principals of the present invention;

FIG. la is a sectional view of the solenoid case of FIG. 1;

FIG. lb is a sectional view of the solenoid armature of FIG. 1;

FIG. 10 is an end view of the cap member of FIG. 1;

FIG. 2 is a plan view of the solenoid spring employed in the assembly ofFIG. 1;

FIG. 2a is a plot of curves and FIGS. 2b and 2c are top end views ofportions of the spring of FIG. 2 useful in describing the operation ofthe spring;

FIG. 3 is a perspective view of a print head assembly employing solenoidassemblies of the type shown in FIG. 1; and

FIG. 4 is a schematic diagram of the solenoid driver circuit.

Referring now to the drawings, the solenoid assembly 10, shown .best inFIG. 1, is comprised of a cylindrical shaped case 11 which is also shownin sectional fashion in FIG. la. The case is a substantially hollowcylindrical member provided with a recessed shoulder aspaced inwardlyfrom its left-hand end and a recessed shoulder 1 1b spaced inwardly fromits right-hand end. The righthand or rearward end is threaded at 11c forreceiving an adjustable threaded closing cap, to be more fullydescribed. An annular groove 11d is positioned immediately adjacent theinnermost end of tapped portion He. The forward or left-hand end ofcasing 11 is further provided with a slot lle through which the leads ofthe solenoid (to be more fully described) extend so as to couple thesolenoid to the peripheral driving circuitry, not shown herein forpurposes of simplicity.

The forward (left-hand) end of case 11 receives a stem member 12 havinga threaded portion for threadedly engaging a tapped mounting holeprovided in the print head assembly, which will be more fully describedhereinbelow. A lock nut 13 threadedly engages threaded portion 12a tofirmly secure the solenoid assembly to the print head assembly. 7

The stem is provided with a circular shaped flange portion 12b which isreceived within the forward end of case 11 whereby the right-handperipheral edge of flange 12b may nominally abut against shoulder 11a.The rearwardmost portion 12c of stem 12 has a diameter which is lessthan both flange 12b and threaded portion 12a with the extremeright-hand end portion 12d being of still further reduced diameter so asto form a shoulder 12c positioned between sections 120 and 12d.

Stem 12 is provided with an axially aligned opening which is comprisedof a portion 12f of a first diameter and a portion 12g of slightlyenlarged diameter extending therethrough. The extreme left-hand endportion 12h is tapered to form a conical entrance portion to facilitatethe insertion of a hollow tubular elongated wire guide 14 which ispositioned within opening portion 12g so that its right edge abutsagainst the shoulder l2j positioned between portions 12f and 12g of theaxially aligned opening. The wire guide is preferably forcefitted withinopening portion 12g so as to experience no linear movement relative tostem 12. The interior of wire guide 14 is preferably coated with alubricant such as, for example, Molycote" to reduce wearing of the printwire 15 which is mounted for reciprocating movement therethrough.

A solenoid core member l6 having a tubular shaped portion 16a and anoutwardly extending flange portion 16b, is telescoped upon theright-hand end of stem 12 whereby the left-hand end of hollowcylindrical portion abuts against shoulder He in the manner shown. Thehollow cylindrical portion [6a is preferably forcefitted upon theright-hand portion 12d of stem 12. The flanges 12!: and 16b, the stemportion 12c, the tubular portion 160 and the interior surface of case 11define a hollow interior space which is provided for receiving thesolenoid coil 17 whose turns or windings are wound about stem portion120 and tubular portion 160 with the axial length of the windings beingdefined and physically limited by flanges 12b and 16b. The two endterminals of solenoid coil 17 are wound so as to extend through the slotHe provided in case 11 (see FIG. la). The coil end terminals 17a and 17bare shown as extending outwardly through slot e. A predetermined lengthof each of the leads 17a and 17b is mounted within an associatedinsulating sleeve 17c and 17d, respectively.

As was described hereinabove, the slender elongated print wire isslidably received by the interior of wire guide 14 and extends stillfurther to the right so as to be slidably received by the portion 12f ofthe stem axial opening and to protrude therebeyond by a predetermineddistance. The right-hand end portion 15a of print wire 15 is mountedwithin an axially aligned opening 18a provided within the solenoidarmature 18. Armature 18 is a substantially cylindrical shaped memberwhich is further provided with a right-hand portion 18b of reduceddiameter relative to the main body portion, forming an annular shoulder18c, portion 18b which is further provided with an axially alignedopening 18d. A solenoid spring 19 is mounted upon armature 18 so thatits central opening '(tobe more fully described in connection with FIG.2) receives reduced diameter portion 18b and rests against shoulder 18c.A washer 20 is positioncd upon spring 19 so that its central openingreceives reduced diameter portion 18b. A fastening member 21 has a shaftportion (not shown)'passing through I the openings in washer 20, spring19 and armature opening 18d so as to firmly secure washer 20 and spring19 to the right-hand end of armature 18.

Spring member 19 has a substantially circular shaped periphery as willbe described in detail herinbelowJA ring shaped spacer member 22 has itsleft-hand surface whose left-hand surface bears against the head offastener 21. An axially aligned opening 230 is provided for adjustingthe preloading of spring 19 in a manner to be more fully described. Alinear groove 23d is provided on the left-hand surface of cap 23 tofacilitate the insertion of an adjusting tool such as, for example, ascrew driver head. By rotating cap 23 within casing 11 the amount ofpreloading of spring 19 may be easily and accurately controlled.

After assembly of the solenoid, the stem 12 may be accurately positionedrelative to casing 11 and maint'ained in this position by application ofa suitable epoxy shown at 24 which, in addition to securing stem 12 tocasing 11, serves to provide a moisture seal over slot l'le'. Ifdesired, a shim in the form of a washer 25 may be positioned betweenshoulder 11a of case 11 and the right-hand peripheral edge of flange12b. As another obvious alternative, the annular periphery of flange 12bmay be threaded and the interior surface of the extreme left-handportion of casing 11 may be tapped so as to provide for adjustablethreaded engagement between these two members. Deposition of epoxy uponthe members in the manner shown at 24 may be provided to rigidly securestem 12 to case 1 l in the desired position.

As was described hereinabove, the amount of preloading imposed uponspring 19 may be controlled by rotation of cap 23 within casing 11. Theamount of preloading may be measured by the insertion of a probe throughaxial opening 23. After precise adjustment of the preloading, epoxy maybedeposited at the position 25 shown in FIG. 1c in order to rigidlymaintain cap 23 in the desired position. From the foregoing it can beseen that the assembly of the solenoid can be performed in a simplestraightforward fashion since relatively little concern need be given tofinal adjustment thereof. For example, the assembly steps may becomprised of mounting core 18 upon stem 12; winding coil 17 about thisassembly and inserting the assembly into casing 11. Spacer 22 may thenbe positioned within casing l 1 so as to abut against shoulder 11b. Thearmature assembly comprised of print wire 15, armature l8, spring 19,washer 20 and fastening member 21 may then be inserted into the casingwhereby wire guide 15 is passed through the axial opening in stem 12(which may be fitted with wire guide 14 either before or after insertionof wire 15 therethrough). Finally, cap 23 may be threaded into casing 11thereby completing the asassembly may then be performed, at which timeepoxy is deposited at the forward and rearward portions ofthe housing inthe manner shown in FIG. 1 to retain the components in their desiredalignment.

FIG. 2 is a detailed view of spring 19 which is comprised of a centralportion having a central opening 19b for receiving the reduced diameterportion 18b of armature 18. A plurality of radially aligned spokeportions 19c are integrally formed with the central portion 19a andextend outwardly therefrom. the free ends of spoke portions 190 are eachprovided with arcuate shaped portions 19d whose outer edges define a substantially circular shaped periphery. The free ends of adjacent arcuateshaped portions define gaps [9e therebetween. As was describedhereinabove, the central portion 19a engages armature 18 while themarginal portions, arcuate sections 19d rest upon the righthand surfaceof spacer 22 (relative to FIG. 1). It has been found that the provisionof gaps 19c between the adjacent free ends of arcuate sections 19dsignificantly increase the operating life of the spring.

In one preferred embodiment of the present invention, the print wiresare moved a distance of 0.015 inches from the rest position to theimpact position. FIG. 2a is a plot which is useful in explaining theoperation of spring 19 wherein distance D is plotted against springforce F. The nature of a flexing member, such as, for example, the spokeportions 19c, is such that the spring force varies linearly withdistance, i.e., F kx, where F is the spring force, x is the flexing ordeflection distance, and k is the spring constant. Curve A representsthe spring force of a flexing member. Due to the design of the spring19, however, it has been found that the relationship of force againstdistance is a logarithmic relationship as represented by curve B of FIG.2a. Although it is not desired to predicate patentability on the theoryof operation of the spring, it is believed that the logarithmicrelationship is the result of the combined characteristics of theflexing and twisting of the spoke and arcuate shaped portions,respectively, which appear to operate as follows:

Considering FIGS. 2b and 2c, which are top and end views respectively ofone spoke 19c and one arcuate shaped portion 19d the energization of thesolenoid coil causes the armature to move toward the left from the restposiion (relative to FIG. 1). The initial movement of armature l8imparts a force represented by arrow F, as shown in FIG. 2c, to beimparted to the spring. The force F initially causes the spokes to flexdownwardly as shown by dotted lines 190". After the spokes undergo apredetermined amount of flexure it has been found that the arcuateshaped portions, such as, for example, arcuate shaped portion 19d,reacts in such a manner that its end portions 19f and 19g remainstationary while its central portion intermediate the ends experiences atwisting relative thereto, as shown by arrow R, which is the apparentreason for the significant departure from the linear force versusdistance relationship to the logarithmic relationship as shown by therespective curves A and B.

When the solenoid coil is deenergized, the magnitude of the spring forceis substantially at its maximum value (represented by the value of curveB when the print wires have moved a distance of the order of 0.015inches) thereby placing the spring wire exclusively under control ofspring 19. The large force imposed upon the armature at this time causesa rapid return of the armature toward the rest position. The spokeportions tend to return to their normal undeflected position. thearcuate portions tend to significantly reduce the amount of overshootingwhich may occur as a result of the movement of the spoke portions towardtheir normal undeflected state thereby significantly reducing the amountof bounce and overshooting which would otherwise occur in springs ofconventional de- In the high speed printer described in the abovementioned copending application Ser. No. 35,405, the print wires aredesigned to move from the initial rest position through a distance of0.015 inches to the impact position and return to the rest positionwithin the span of l millisecond. Since a typicalline may contain 132characters, any given print wire may experience as many as 660deflections per line, all of said deflections occurring within the spanof less than 1 second. Taking the above high speed operationinto-consideration with the fact that the printer is intended for useover extremely long periods of time with a minimum of down time, it canbe seen that the spring members must be quite rugged while at the sametime providing highly uniform operating characteristics for any givenindividual deflection. This necessitates the provision of a springmember which is dimensionally precise.

FIG. 3 is an exploded perspective view showing a printing head assembly30 which incorporates sole- I noids 19 of the type describedhereinabove. The assembly 30 is comprised of a body portion 31 having arear wall 32 provided with a plurality of tapped openings 33 whichthreadedly engage the threaded portions 12a of each of the solenoidstems 12.

Body portion 31 is further provided with first and second pairs ofvertically aligned guide slots 34 and 35 for receiving positioningmembers 36 and 37 which are respectively positioned within the guideslots to align the slender print wires therethrough. A plurality of wireguides 38 extend through appropriate openings provided in positioningmembers 36 and 37m guide the print wires therethrough. The interiors ofth hollow elongated tubular members 38 are preferably lubricated toreduce wear.

The forward end of housing 31 is provided with an opening 39 and a pairof guide slots 43a and 43b for receiving and positioning a jeweledmember 44 which, in turn, is provided with a plurality of verticallyaligned openings for receiving each of the print wires.

Housing 31 is further provided with a pair of outwardly extendingflanges 44a and 44 b each having tapped openings 45 for threadedlyengaging fastening members to secure the print head housing to mountingplates 46a and 46b which form a part of the printer and serve to movethe print head along a horizontal line to successively print a pluralityof vertical columns of dots, with the total number of dots per columnbeing selectively controlled in accordance with the particular characteror symbol to be printed.

The rear end of housing 31 further supports a registration device 60shown in FIG. 3a and includes a pair of supporting members 61,61 forsecuring assembly 60 to housing 32. A plate 63 is secured to members61,61 and supports a lamp 64 and a photo-head 65 suspended from theunderside of plate 63. A stationary mylar strip 66 having a plurality ofnarrow transparent slits 67 is mounted in spaced parallel fashionrelative to the printer platen (not shown). As the print head assemblymoves in the direction of arrow A during a printing operation, lightpasses through a transparent slit and inpinges on the photo-head 65. Thelight is carried by a fiber optic bundle 68 to a photocell 69 toactivate the photocell only when passing a transparent slit. thephotocell generates a pulse which enables the solesolenoids to beenergized only at the positions of the narrow transparent slits thusassuring accuracy in the locations of the dots printed regardless of thespeed of movement (i.e., at a constant or a non-constant speed) of theprint head assembly.

FIG. 4 is a schematic diagram showing the driver circuitry employed forselectively driving solenoids of the type shown in FIG. 1. Since thedriving circuits for each solenoid are all identical in both design andoperation,

' only one of the circuits will be described herein for purposes ofsimplicity.

As shown in FIG. 4, the driving circuit 50 is provided with an inputterminal 51 normally coupled to one of the seven outputs of a charactergenerator of the type described in the aforementioned application Ser.No. 35,405. when the output of the character generator coupled to inputterminal 51 is low, input terminal 51 is held at ground potential and nocurrent flows through resistor R4. When the output of the charactergenerator goes high, then current flows through resistor R4. If diode D1is back-biased, the current flows into the base of transistor Q3. Thiscurrent, in turn, causes current to flow through resistor R3 andtransistor Q3, thereby establishing a voltage across Zener diode ZDI.This voltage causes transistors Q2 and O1 to act as emitter followers,thereby developing a voltage drop across resistor R1. Resistor R2 limitsthe power dissipation in transistor Q2. The voltage drop across resistorR1 causes a collector current of the order of 2.5 amps to flow throughtransistor Q1. When the stage first turns ON, the inductance of thesolenoid prohibits the current from flowing through transistor Q1 and atthis point transistor O1 is saturated. When the current flowing throughthe solenoid reaches approximately 2.5

amps, transistor Q1 goes into an active region and limits the current atthis value.

When transistor 01 shuts OFF, the current flowing through the solenoidpasses through diode D2 and causes capacitor C1 to charge. The value ofcapacitor C1 is chosen to act as a parallel resonant circuit with theinductance in the solenoid. Diode D allows only one-quarter wave of thischaracteristic fr quency to be developed thereacross.

After the capacitors of each charging circuit (one charging circuit ofthe type 50 shown in FIG. 4 is provided in each solenoid) have chargedto approximately 60 volts due to the discharge of the printer solenoids,resistor R5 serves to bleed ofi' this charge so that the capacitors willbe at a 30 volt bias at the time of the next discharge. The value of theresistor is chosen so that a time constant of about 275microsecondsresults.

Each diode D1 of each of the driver circuits of the type 50 shown inFIG. 4 have their cathodes tied together and connected to the collectorof transistor Q4 in the manner shown. When the printer is normallyrunning, a voltage is developed across relay S1 to close its relaycontact Cs to prohibit current from flowing through diode D3 and intothe base of transistor Q4, thereby insuring that transistor Q4 is shutoff. In this condition, each diode D1 of each driver circuit cannotshunt current away from the solenoid drivers. When the printer is shutoff, however, the power supply which energizes relay Rl will drop beforethe power supply employed to operate the solenoid driver circuits 50.When this happens, relay R1 opens, thereby allowing the current to flowthrough resistor R6 and diode D3 into the base of transistor Q4, therebysaturating 04. This causes all the bases of the solenoid drivers to betied to ground through their diodes D1, thus preventing any of thesolenoids from firing on shut off. As shown in FIG. 4, each solenoid andsolenoid driver circuit is protected against overload current by a fuseF1.

Although not shown for purposes of simplicity, it should be understoodthat a cover merlnber is positioned upon housing 31 so as to keep theinterior of housing member 31 free from contamination.

lt can be seen from the foregoing description that the present inventiondescribed a solenoid assembly especially adapted for use in printers ofthe dot matrix type in whch high speed of th print wires comparable withpresent day printers are obtained due to the nature of the design of thesolenoid assembly and of the springs employed therein which are furtherdesigned to provide a unique operation not heretofore obtained in theperformance of such components.

Although there has been described a preferred 'embodiment of this novelinvention, many variations and modifications will now be apparent tothose skilled in the art. Therefore, this invention is to be limited,not by the specific disclosure herein, but only by the appending claims.

What is claimed is:

I l. A driver circuit for energizing the coil of a solenoid comprising:

a d.c. source;

a capacitor, a first diode and said coil being connected in seriesacross said do. source with one terminal of said first diode connectedto one terminal of said capacitor,

a resistor being connected in parallel across the series connected coiland diode;

first transistor means having base, emitter and collector electrodes;

a second resistor coupled to said emitter; said electrode and saidsecond resistor being connected in parallel across the series connectedfirst diode and capacitor;

second transistor means having an input and at least one outputterminal;

said one output terminal being connected to the base of said firsttransistor means;

voltage regulating diode means connected to the input of said secondtransistor means for controlling the voltage applied to the baseelectrode of said first transistor means when said second transistormeans is rendered conductive to thereby supply constant current to saidcoil; said diode coupling said coil to said capacitor when said firsttransistor means is rendered non-inductive.

2. The driver circuit of claim 1 wherein said capacitor and the solenoidcoil form a resonant circuit; said first diode being adapted to passonly one-quarter wave of the characteristic frequency to said capacitorwhen said first transistor means is turned off.

3. The driver circuit of claim 1 further comprising second diode meanscoupled to the input of said second transistor means;

third transistor means for back-biasing said second diode means whensaid third transistor means is not conducting;

a second d.c. source;'

means responsive to turn off said second d.c. source for turning on saidthird transistor means to backbias said second diode means to preventsaid coil from being erroneously energized by said capacitOl'.

1. A driver circuit for energizing the coil of a solenoid comprising: ad.c. source; a capacitor, a first diode and said coil being connected inseries across said d.c. source with one terminal of said first diodeconnected to one terminal of said capacitor, a resistor being connectedin parallel across the series connected coil and diode; first transistormeans having base, emitter and collector electrodes; a second resistorcoupled to said emitter; said electrode and said second resistor beingconnected in parallel across the series connected first diode andcapacitor; second transistor means having an input and at least oneoutput terminal; said one output terminal being connected to the base ofsaid first transistor means; voltage regulating diode means connected tothe input of said second transistor means for controlling the voltageapplied to the base electrode of said first transistor means when saidsecond transistor means is rendered conductive to thereby supplyconstant current to said coil; said diode coupling said coil to saidcapacitor when said first transistor means is rendered non-inductive. 2.The driver circuit of claim 1 wherein said capacitor and the solenoidcoil form a resonant circuit; said first diode being adapted to passonly one-quarter wave of the characteristic frequency to said capacitorwhen said first transistor means is turned off.
 3. The driver circuit ofclaim 1 further comprising second diode means coupled to the input ofsaid second transistor means; third transistor means for back-biasingsaid second diode means when said third transistor means is notconducting; a second d.c. source; means responsive to turn off saidsecond d.c. source for turning on said third transistor means toback-bias said second diode means to prevent said coil from beingerroneously energized by said capacitor.